This invention relates generally to camshafts of internal-combustion engines and more particularly to built-up camshafts of the type wherein a hollow structure such as a tube is used for the shaft, and separate structures such as cam parts and shaft end members are fixed securely to the shaft by a process such as bulging to obtain an integral built-up camshaft.
Built-up camshafts of the instant character produced by a bulging are known in the art, examples of which are disclosed in Japanese Utility Model Application Laid-Open Publication No. 124205/1981, and Utility Model Publication No. 966/1981. Furthermore, built-up camshafts in which shaft end members are attached to the ends of a hollow shaft are disclosed in publications such as Japanese Utility Model Application Laid-Open Publication Nos. 1704/1983 and 73914/1983.
Built-up camshafts have heretofore been produced by a process wherein, after cam plates or disks and a hollow shaft have been integrally joined by a bulging process, shaft end members are fitted onto the two ends of the hollow shaft, and, in order to prevent relative rotation and axial movement between the hollow shaft and the shaft end members, they have been fixed together by a measure such as welding or brazing. As a consequence, the required amount of labor is high, whereby the production is disadvantageously inefficient.
Furthermore, when the cam disks are fixed to the hollow shaft by a bulging process, specific bulges are formed in the hollow shaft, and the overall length of the hollow shaft becomes shorter. In this case, great deviations occur in the dimensions from the shaft ends to the adjacent cam disks between camshafts that have undergone the bulging process. Accordingly, in order to obtain accurate dimensions between the cam disks and the shaft end members and between the two shaft end members, it is necessary to carry out dimensional adjustments of the shaft end part of the hollow shaft after the bulging process and before the attachment of the shaft end members.
Another problem encountered heretofore is that, when the shaft end members are attached to the ends of the hollow shaft by a bulging process, it is difficult to obtain a strong joint therebetween. Furthermore, the reliability of the seal between the mutually contacting parts of the hollow shaft and the shaft end members during this attachment is low, and for this reason deviations in the strength of the joints of the shaft end members of the completed built-up camshafts occur and leave uncertainty as to the product quality.
On this point, it is not possible at all to determine from an inspection of the exterior the state of the joint between a hollow shaft and the shaft end members attached thereto, and, moreover, a nondestructive inspection of the state of the joint is difficult. Accordingly, there has been a need for elevating the reliability of the joint strength.
The strength of the joints between the hollow shaft and the shaft end members can be increased by raising the hydraulic pressure in the bulging process, but, because this gives rise to breakage of cam disk, there is a limit to the hydraulic pressure which can be applied. However, with a hydraulic pressure of a low level such as to avoid breakage of the cam disks, the reliability of the joint strength as described above has been deficient.